Weaning off of longer halflife drugs will always be a smoother withdraw. Some xanax users actually use long acting benzos to wean and minimize their withdrawals. As for dose, two drugs of two different half lives, just expect the oxycodone to not last as long as the morphine ER. Say, if you expect your morphine to relieve pain 8 hours, Expect an instant release drug, such as using the equivalent strength of oxycodone in IR form, expect to have much better pain relief results but a shorter half life, so just ~4hours maybe of great relief rather than 8 hours of mild relief.